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.\" Copyright (C) 2000 Jason Evans <jasone@FreeBSD.org>.
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.\" $FreeBSD: src/lib/libc_r/man/sem_wait.3,v 1.8 2001/10/01 16:09:09 ru Exp $
.Dd $Mdocdate: June 5 2013 $
.Dt SEM_WAIT 3
.Os
.Sh NAME
.Nm sem_wait ,
.Nm sem_timedwait ,
.Nm sem_trywait
.Nd decrement (lock) a semaphore
.Sh SYNOPSIS
.In semaphore.h
.Ft int
.Fn sem_wait "sem_t *sem"
.Ft int
.Fn sem_timedwait "sem_t *sem" "const struct timespec *abstime"
.Ft int
.Fn sem_trywait "sem_t *sem"
.Sh DESCRIPTION
The
.Fn sem_wait
function decrements (locks) the semaphore pointed to by
.Fa sem ,
but blocks if the value of
.Fa sem
is zero, until the value is non-zero and the value can be decremented.
.Pp
The
.Fn sem_timedwait
function decrements (locks) the semaphore pointed to by
.Fa sem ,
but blocks if the value of
.Fa sem
is zero, until either the value is non-zero and can be decremented
or the system time specified by
.Fa abstime
is reached.
.Pp
The
.Fn sem_trywait
function decrements (locks) the semaphore pointed to by
.Fa sem
only if the value is non-zero.
Otherwise, the semaphore is not decremented and
an error is returned.
.Sh RETURN VALUES
.Rv -std sem_wait sem_timedwait sem_trywait
.Sh ERRORS
.Fn sem_wait ,
.Fn sem_timedwait ,
and
.Fn sem_trywait
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa sem
points to an invalid semaphore.
.El
.Pp
Additionally,
.Fn sem_timedwait
will fail if:
.Bl -tag -width Er
.It Bq Er ETIMEDOUT
The semaphore value was zero and could not be decremented before
.Fa abstime
was reached.
.El
.Pp
Additionally,
.Fn sem_trywait
will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The semaphore value was zero, and thus could not be decremented.
.El
.Sh SEE ALSO
.Xr sem_destroy 3 ,
.Xr sem_getvalue 3 ,
.Xr sem_init 3 ,
.Xr sem_open 3 ,
.Xr sem_post 3
.Sh STANDARDS
.Fn sem_wait ,
.Fn sem_timedwait ,
and
.Fn sem_trywait
conform to
.St -p1003.1-2008 .
